A warm water pool in the Indo-Pacific Ocean that influences rainfall in many parts of the world, including India, is adding a ‘California’ to its size every year. In the process, it is slowly but steadily altering rainfall patterns in many regions, a study led by an Indian scientist showed on Wednesday.

The Indo-Pacific warm pool has been warming rapidly and expanding during the recent decades in response to an increase in carbon emissions due to climate change. It has already grown to double its size, from an area of 22 million sq km during 1900-1980 to 40 million sq km during 1981-2018. The annual rate of expansion of late has been 4,00,000 sq km, an area about the size of California, said an international team of meteorologists led by Roxy Mathew Koll of the Indian Institute of Tropical Meteorology (IITM), Pune.

This two-fold expansion in the warm pool — the largest expanse of the warmest waters on earth — is influencing the most dominant mode of weather fluctuations originating in the tropics, called the Madden-Julian Oscillation (MJO). The MJO is an ocean–atmosphere coupled phenomenon, represented by a band of rain-bearing clouds moving eastward over the tropics.

What the MJO does

It regulates tropical cyclones, the monsoons, and the El Nino cycle, and occasionally contributes to severe weather events over Asia, Australia, Africa, Europe and the Americas. These clouds travel a stretch of 12,000-20,000 km over the tropical oceans, mainly over the Indo-Pacific warm pool, which has ocean temperatures generally warmer than 28°C. The team, which included Koll’s research student Panini Dasgupta and his peers from the US National Oceanic and Atmospheric Administration, universities of Washington and Tokyo, found that the changes in MJO behaviour have increased the rainfall over North Australia, West Pacific, Amazon Basin, South-West Africa and South-East Asia, but negatively impacted rainfall in North India, the Central Pacific, East and West coast of the US and East Africa, among others. Over North India, the decline in rainfall was mainly during the winter-spring season — between November and April.

“Climate model simulations indicate that continued warming of the Indo-Pacific Ocean is highly likely, which may further intensify these changes in global rainfall patterns in the future,” said Koll. “Much of these changes will be witnessed over the countries in the tropics. As these countries currently have very poor forecasting systems for severe weather events, there is an urgent need to create them,” he told BusinessLine.

Weather changes

The scientists showed that because of this warm pool expansion, the residence time of MJO clouds have shortened over the Indian Ocean by four days (from an average of 19 days to 15 days). Over the West Pacific, it increased by five days (from an average of 18 days to 23 days). It is this change in the residence time of MJO clouds that has altered the weather patterns across the globe, they argued.

“I am fascinated by the results because it seems like an increase in MJO residence time over the Maritime Continent has occurred with a decrease over the Indian Ocean,” said Raghu Murtugudde, an atmospheric scientist with the University of Maryland. The Maritime Continent, the name given to South-East Asia by meteorologists because the region gets copious rain all round the year, is the strongest heat source in the world. It is hence also called the boiler box and the mix master because of the heating of atmospheric convection and confluence of various currents in the ocean.

What does this mean for the monsoon whose intra-seasonal variability is out of phase with the MJOs, wondered Murtugudde. MJOs happen during October-April and monsoon intra-seasonal oscillation during May-September, he pointed out.